Probably because they bothered to pursue such a thing and hundreds of millions people did not.
How do you conclusively know if someone's content generating machine infringe with your rights? By saving all of its input/output for investigation.
It's ridiculous, sure but is it less ridiculous than AI companies claiming that the copyrights shouldn't apply to them because it will be bad for their business?
IMHO those are just growth pain. Back in the day people used to believe that the law don't apply on them because they did it on the internet and they were mostly right because the laws were made for another age. Eventually the laws both for criminal stuff and copyright caught up. Will be the same for AI, now we are in the wild west age of AI.